Three dominant legume species (Tephrosia virginiana (L.) Pers., Rhynchosia reniformis DC., and Centrosema virginianum (L.) Benth.) found in frequently burned longleaf pine woodlands were grown in a garden plot and treated with a factorial arrangement of shade treatments (±) and burning (±) treatments. Nitrogen fixation was measured using 15N Isotope dilution method with whole plant recovery (including root systems). Burning treatments decreased total biomass (P < 0.001) and increased root to shoot ratios (P < 0.001). N2-fixation rates were generally reduced by burning, but there was a complex interaction among species, shade, and burn treatments. For Tephrosia, contrasts of shade (F1,42 4.48, P < 0.05) and burn (F1,42 4.42, P < 0.05) indicate that both main treatment effects significantly reduced N2-fixation rates. Relative to all other treatment combinations, Rhynchosia had elevated levels of N2-fixation under the no shade by burn treatment. Centrosema showed no effect of shade on N2-fixation (F1,42 1.65, P > 0.05), but burns significantly lowered N2-fixation rates for both shaded and unshaded plants (F1,42 19.31, P < 0.001). Despite the diverse response of species N2-fixation activity to treatments, a regression of all individuals regardless of species shows a curvilinear relationship of % N derived from the atmosphere to total plant biomass. The diverse response of these herbaceous legumes to light and fire challenges the conventional wisdom that legumes maintain functional redundancy as a guild across environmental gradients within longleaf pine woodlands.
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Vol. 134 • No. 3